Cranial anatomy of Hybodus basanus Egerton from the Lower Cretaceous of England. American Museum novitates ; no. 2758

Supplemental Materials
Journal Title
Journal ISSN
Volume Title
New York, N.Y. : American Museum of Natural History
"A detailed revision of the cranial anatomy of Hybodus basanus is presented. Features of the cranial and visceral endoskeleton and associated dermal elements are described. The neurocranium differs from that of modern and other fossil sharks, notably in the ethmoid and otico-occipital regions. The ethmoid regions of Hybodus and Xenacanthus are shown to be similar in some respects. The otico-occipital regions of Hybodus and Xenacanthus differ profoundly in the topographical arrangement of the otic capsule, postorbital process, and lateral otic process. Topologically, the arrangement in Hybodus could be obtained by telescoping the otic capsules, lateral otic processes, and occiput of a Xenacanthus braincase between its postorbital processes. This imagined telescoping produces a 'short' otic region in Hybodus, but the arrangement differs from that found in modern sharks. The jaws of Hybodus basanus have a strong ethmoidal articulation but lack a postorbital one; limited protraction of the jaws seems to have been possible. There are five large labial cartilages in each cheek. In some respects these labial cartilages resemble those of chimaeras, and various theories of homology between shark and chimaeroid labial cartilages are reviewed. The hyoid arch is peculiar in that the hyomandibula passes dorsal to the otic ramus of the palatoquadrate. Five branchial arches are present, but only their epi- and ceratobranchials are described. A complete dentition is described and variation between the teeth is discussed; variation seems sufficient to include H. ensis and H. parvidens as synonyms of H. basanus. Dermal scales from various parts of the head and mouth are also described. Although the absence of critical data for most fossil sharks makes it difficult to establish a testable hypothesis of relationship, an attempt is made to compare H. basanus with other sharks. Hybodus basanus is the only Mesozoic shark in which the cranial anatomy has been studied, and the author cautions other systematists against attaching too much weight to the notion that Hybodus is closely related to modern sharks"--P. 2.
64 p. : ill., map ; 26 cm.
Includes bibliographical references (p. 59-64).